1. Field of the Invention
The present invention relates to a temperature compensated long period optical fiber grating filter.
2. Description of the Related Art
Generally, a long period optical fiber grating filter is an optical device for coupling modes propagated in the core of an optical fiber to modes propagated in the cladding of the optical fiber. Such a long period optical fiber grating filter provides an advantage in terms of the gain flatness of erbium-doped optical fiber amplifiers (EDFAs) in that it is of a mode-coupling type other than a reflective mode-coupling type. Such a long period optical fiber grating is manufactured to exhibit a periodic variation in refractive index at its core. The periodic variation in refractive index is obtained by periodically exposing the core of an optical fiber, sensitive to ultraviolet rays, in the optical fiber grating to ultraviolet rays in the process of manufacturing the optical fiber grating. That is, an increase in refractive index is exhibited at portions of the core exposed to ultraviolet rays whereas no variation in refractive index occurs at the remaining portions of the core not exposed to ultraviolet rays. Thus, a periodic variation in refractive index is exhibited in the core. In such a long period optical fiber grating, a mode coupling occurs in a state in which a phase matching condition expressed by the following Expression 1 is satisfied. EQU .beta..sub.co -.beta..sub.cl.sup.(m) =2.pi./.LAMBDA. [EXPRESSION 1]
where, .beta..sub.co represents a propagation constant of a core mode, .beta..sub.cl represents a propagation constant of an m-th cladding mode, and A represents a grating period.
Where .beta.=2.pi.n/.lambda. is substituted into Expression 1 ("n" represents a refractive index, and .lambda. represents a wavelength), the refractive index difference between the core and cladding modes is derived which corresponds to n.sub.co -n.sub.cl.sup.(m) =.lambda./.LAMBDA.. Accordingly, a change of light with a certain wavelength into a cladding mode can be achieved by appropriately determining a desired grating period .LAMBDA. and a desired refractive index difference n.sub.co -n.sub.cl.sup.(m).
A desired refractive index difference can be obtained by appropriately radiating an ultraviolet laser onto the optical fiber which is sensitive to ultraviolet rays. That is, the optical fiber sensitive to ultraviolet rays is masked by a mask having a particular period. When a laser is then radiated onto the mask, the opto-sensitive optical fiber generates a reaction resulting in a variation in the refractive index of the core. In order to obtain a desired spectrum, that is, a desired coupling wavelength and a desired extinction ratio, the radiation of the ultraviolet laser should be carried out for an appropriate period of time by accurately adjusting the mask period.
The coupling wavelength of the long period optical fiber grating manufactured as mentioned above is also influenced by temperature. A shift of coupling wavelength depending on a variation in temperature is based on a variation in refractive index depending on the temperature variation and a thermal expansion in length depending on the temperature variation. This can be expressed by the following Expression 2: ##EQU1##
where, T represents a temperature.
Where a long period optical fiber grating is applied to general optical fibers for communication or dispersion shifted optical fibers, the second term of the right side in Expression 2 is not taken into consideration because the value defined by the first term of the right side in Expression 2 is greater than the value defined by the second term by about several ten times. For instance, the Flexcor 1060 manufactured by Corning Glass Corporation exhibits a coupling wavelength of about 5 nm per 100 C. Typical dispersion shifted optical fibers exhibit a coupling wavelength shift of about 0.3 nm per 100 C due to a variation in refractive index occurs while exhibiting a coupling wavelength shift of about 5 mn per 100 C due to a length expansion. In the case of a gain flattening filter, which is an example of a practical application of long period optical fiber gratings, however, a temperature stability of about 0.3 nm per 100 C is required.
In order to obtain a temperature compensation meeting the above requirement, a method has been used in which the refractive index of the filter is adjusted in such a fashion that the term d.lambda./d.LAMBDA. in Expression 2 has a negative value. There is another conventional method in which the period of the long period optical fiber grating is shortened in such a fashion that a higher-order cladding mode is selected. Another method is also known in which an addition of B.sub.2 O.sub.3 is made to allow the term "dn/dT" in Expression 2 to have a value of 0.
However, all the above mentioned conventional methods use complicated processes because they involve a refractive index adjustment in the filter or an addition of a material serving to avoid a variation in refractive index caused by a variation in temperature. U.S. Pat. No. 5,757,540 for Long-Period Fiber Grating Devices Packaged For Temperature Stability to Judkins et al discloses a material package surrounding the cladding about the long-period grating for temperature stability. However, Judkins et al '540 does not disclose coating the region of the optical fiber cladding absent from a long period grating. What is needed is an optical fiber with two separate coatings, one for the region containing the long period grating and the other for the region absent the long period grating. This would result in an optical fiber with a uniform diameter in all regions which is easier to handle and use.